1. Field of the Invention
The present invention relates to DC power supply systems that step-up DC voltage of a DC power supply such as a battery to DC voltage having a neutral point, the DC power supply systems including uninterruptible power systems (UPSs), solar power generation systems, and fuel cells, for example. The invention relates in particular to a control system to keep balance between the voltages across a capacitor and across another in a series-connected capacitor circuit provided in the DC output side of the DC power supply system.
2. Description of the Related Art
FIG. 8 is a circuit diagram of the DC power supply system employing conventional technology disclosed in Japanese Unexamined Patent Application Publication No. 2008-295228. The main circuit comprises a DC power supply 8 for supplying DC power, reactor 1, switching elements 2A and 2B, diodes 3AD and 3BD, and output capacitors 4A and 4B. The switching elements 2A and 2B perform a switching operation to obtain desired voltages across the capacitor 4A and across the capacitor 4B, supplying the load 9 with a DC output voltage higher than the voltage of the DC power supply 8. The control operation of this circuit construction is performed by ON-OFF-controlling the switching elements 2A and 2B with shifted phase angles using the reactor 1 composed of two magnetically coupled windings. This control system reduces ripples in the current through the reactor 1, forming a device in a small size and at a low cost, which is the aim of the control system. Details of the control system are described in Japanese Unexamined Patent Application Publication No. 2008-295228 and so omitted here. This circuit is a unidirectional step-up chopper circuit for supplying power from the DC power supply 8 to the load 9. The circuit, however, becomes also a bidirectional step-up chopper circuit for regenerating power from the load 9 to the DC power supply 8 by adding switching elements anti-parallel-connected to the diodes 3AD and 3BD.
Also shown in FIG. 8 are reactor nodes 1A, 1B, 1C and 1D, diodes 2AD and 2BD, and pulse width modulation circuits 16 and 19 that receive inputs from a control circuit 15. Circuit 16 includes a comparator 18 with an input carrier generator 17, and circuit 19 includes a comparator 21 with an input carrier generator 20. Also shown in FIG. 8 are voltage detection circuits 11, 13 and 14, and circuit elements 10 and 12. Voltage detection circuits 13 and 14 are connected at a node 5.
When the chopper of Japanese Unexamined Patent Application Publication No. 2008-295228 is connected to a three-level inverter such as an uninterruptible power system (UPS), the inverter, a DC to AC conversion circuit, is connected to the chopper at both ends of the capacitors 4A and 4B and the point of series connection between the two capacitors. The voltages across the capacitor 4A and the voltage across the capacitor 4B become unbalanced due to dispersion in the voltage drops of semiconductor elements, the impedances of circuit components, and gate signals, and also due to imbalance of DC power caused by the inverter as a load. This imbalance results in excessive voltage or deficient voltage that may cause interruption of operation.